There are two types of flash memory devices, those based on NOR gates (NOR flash memory) and those based on NAND gates (NAND flash memory). A modern NOR memory cell may employ two gates, a control gate and a floating gate. The charge held in the floating gate partially screens the electric field from the control gate to establish and control the threshold voltage of the cell. Depending on the threshold voltage of the cell, a read-out voltage applied to the control gate will either result in a conducting channel (e.g., producing a current) or an insulated channel (e.g., producing no current). Stored data can then be interpreted from the absence or presence of a current. In modern NOR flash, the default state is a logical “1” because a current flows in the default state. The cell can be programmed to a “0” value via hot-electron injection of the floating gate and erased by applying a large voltage of opposite polarity. Modern NOR flash is partitioned into erase block such that the entire block must be erased to erase a single cell within the block. Accordingly, a rewrite programming or erasing operation incurs significant penalty. Modern NAND flash may employ tunnel injection to write data to a cell and tunnel release to erase data from a cell.
In typical NOR flash, cells are connected in parallel to the bit lines. Thus, cells can be read and programmed individually. Accordingly, reading from NOR flash is similar to reading from random access memory (RAM). Thus, NOR flash is suitable for software storage and execution similar to read only memory (ROM) and RAM. In typical NAND flash, the cells are connected in series to the bit lines. This prevents the cells from being read and programmed individually. Rather, the cells in the series must be read together at once. The series connection enables large grids of NAND flash memory cells in a small area resulting in high density memory suitable for mass storage. Programming and reading is performed on a page basis enabling large data transfers of blocks of data similar to hard disks.